CROLL, RP*; ROBERTSON, GN; MCGEE, CAS; SMITH, FM; Dalhousie Univ; Dalhousie Univ; Dalhousie Univ; Dalhousie Univ: Development of the swimbladder and its innervation in the zebrafish (Danio rerio)

Gas-filled swimbladders play essential roles in buoyancy control of many teleosts, yet little is known of how these organs and their innervation develop to meet changing demands through ontogeny. Here, we examined the development and time course of innervation of the zebrafish swimbladder. This organ originates as a single chamber from an evagination of the dorsal esophagus and is inflated to a prolate ellipsoid shape by 3-5 days post fertilization (dpf). Acetylcholinesterase histochemistry and zn12 immunoreactivity revealed that the earliest innervation of this organ, occurring at 4-5 dpf, consisted of two bundles of axons extending along the lateral margins. By around 20 dpf the swimbladder developed a second chamber anterior to the first, thus assuming the adult configuration; anterior and posterior chambers were connected through the ductus communicans, while the posterior chamber retained its connection to the esophagus via the pneumatic duct. By this time innervation to the posterior chamber became both more extensive and neurochemically complex. Fibres exhibiting tyrosine hydroxylase immunoreactivity were detected by 20 dpf, and within another 7 d, a mesh of fine vasointestinal polypeptide immunoreactive nerve fibers surrounded the anterior chamber. The basic features of the adult swimbladder morphology and patterns of its innervation were therefore present within the first 30 dpf. Morphometric data demonstrate volumetric contributions of the swimbladder to buoyancy of the developing zebrafish. Together these studies provide insight into how the swimbladder develops and contributes functionally to buoyancy control as the zebrafish matures from its larval to adult form. (Funded by Canadian Space Agency contract No. 9F007-046016/001/ST to RPC and FMS.)